The present invention relates to probe pins used in automatic testing apparatus for printed circuit boards. Each of a plurality of probe pins is mounted in alignment with a selected solder pad or other circuit node of the printed circuit board to be tested. The mounted pins, which are connected at one end to a test circuit, are positioned so that the contact face at the end of the pins are in slight pressure engagement with their respective aligned solder pads or nodes to provide the required electrical contact.
A typical printed circuit board is made up of a sheet of insulating material, such as epoxy-glass laminate, for example, with a plurality of miniature electronic components, such as resistors, capacitors, transistors, etc. mounted on one or both sides of the board. The wire connections for each of the components extend through holes to the opposite side of the board and are connected to each other in the required manner by metallic strips etched on the opposite surface of the insulating board. At the point where the connecting wires for the components extend through the boards, small, thicker and irregular solder pads are formed as part of the metallic strips. In some instances, the tip of the wire extends through the surface of the solder pad; in other instances, the tip of the wire may be bent such that the solder pad completely covers it. To protect the etched surface, the board is usually sprayed with a thin coating of insulating material which is referred to as a conformal coating.
The various components of printed circuit boards were tested in the past, with hand held probes connected at one end to a voltmeter or other instrument. These probes were manipulated manually until the outer end or contact face made the required electrical contact with the desired solder pad. Presently, particularly in the automatic testing of printed circuit boards, a number of probe pins are mounted in a test board; and positioned so that when the test board is placed in close proximity to the opposite or back side of the printed circuit board, the tip or contact face of each probe pin engages the required solder pad to provide the necessary electrical contact. The test circuit is connected electrically to the other end of each probe pin or to a socket in the test board into which each of the pins are mounted. The test circuit may be connected to a computer to provide a diagnostic printout of the entire printed circuit. In such automatic testing apparatus, it is common practice to provide a probe pin which is spring loaded to yield axially at one or two ounces of force, for example, in order to make a proper electrical contact between the contact face of the probe pin and the solder pad. Thus, any slight parallel misalignment between the printed circuit board and the test board into which the probe pins are mounted, or variation in thickness and shape of the individual solder pads does not effect the accuracy of the test.
However, because of the irregular configuration of the solder pads, there are times when the contact face of the probe pin makes a point contact with the solder pad which creates a high resistance, resulting in a faulty test readout. Various attempts have been made to assure an adequate low resistance contact between the contact face of the probe pin and the solder pad by configuring the metallic contact face of the probe pin into a cup, or a small disc or cylinder with a serrated edge, for example, Although these different configurations of the probe pin contact face provided an increase in reliability of the automatic testing equipment, the irregularities and the different configurations of the solder pads still presented problems of high resistance contact. The problem is still more acute with printed circuit boards that have been in service; because, they have been coated with a conformal coating. Of course, an attempt is made to remove the conformal insulative coating from the solder pads by light abrading, so as not to remove any of the etched circuit. However, in some instances, the irregular solder pad surface prevents the complete removal of the coating; and regardless of the configuration of the contact face of the probe pin, an error would occur in the diagnostic readout, either due to a high resistance point contact or failure of the contact face of the probe pin to make contact at all.
One type of probe pin in use, which overcomes the problems of high resistance or improper contact, utilizes a contact face made of a small cylinder of fine woven wire. The woven wire cylinder varies from 1/16 to 1/8 of an inch in diameter, and from 3/100 to 1/10 of an inch in thickness. The diameter of the individual woven wire strands is approximately 3/1000 of an inch. This woven wire contact face of the probe pin appears to the naked eye not unlike a miniaturized metallic scouring pad. Probe pins having the woven metallic wire contact face provide a low resistance area contact and appear to be reliable for the various solder pad configurations and irregularities. However, the woven wire contact probe pins are expensive to fabricate because of the overall small diameter of the contact face and the small diameter of the wire. Further, after repeated deformation during use, wire tends to lose it s resilient aspects, which limits the useful life of the contact face of the probe pin.
The purpose of the present invention is to provide an improved probe pin which is inexpensive to fabricate, has a long life and still retains the advantage of providing at its contact face a low resistance area contact when in slight pressure engagement with printed circuit board solder pads. Such a probe pin should have the aforesaid features to be reliable during automatic testing regardless of the irregularities and different configurations of the solder pads and regardless of traces of the insulated conformal coating which may exist on portions of the solder pads.